I've bought recently the licence for photoshop CC2018 installed on a Mac hig sierra, when I save a picture for web (export-save for web) and try to send it, after a while, comes out the writing: "has occurred an error" and it's not possible to charge it. but if I transfer the same file on a memory key an put it in an other Mac (OS X 10.9.5) and try to send it, that works properly, did I made any error, is there any setting i can do to solve the problem?

#1 Sounds like you're using the HTML options which you should never use. You want to save IMAGES only.

#2 I can't recommend the legacy Save for Web feature anymore. It's really only there for saving animated GIFs. For all other web images I use File > Export > Export As. Select JPG and reduce Quality to reduce file size. See screenshot.